Engines may utilize humidity sensors at various locations for controlling engine operation, such as for controlling spark timing and exhaust gas recirculation (EGR). Errors in the humidity reading can lead to improper spark timing, and thus knock, as well as various other issues with regard to condensate formation, EGR control, diluent control, etc.
US 20120227714 describes an approach with a humidity sensor positioned downstream of an EGR inlet. To diagnose operation of the sensor, the disclosed method closes the EGR throttle valve, and indicates humidity sensor degradation based on each of a change in intake air relative humidity and pressure responsive to the EGR throttle valve closing.
The Inventors have recognized a problem with placing the humidity sensor near the crankcase. Positioning the humidity sensor in such a location may cause mis-reading of ambient humidity due to contamination by crankcase gasses. Further, the humidity of the circulated exhaust gas has a varying humidity based on fuel type, and engine operating conditions, such as temperature.
In one example, some of the above issues may be addressed by a method comprising, while flowing exhaust gasses into an engine intake air downstream of a humidity sensor and upstream of a compressor, indicating humidity sensor degradation when humidity readings of the sensor change by less than a first threshold while pressure at the sensor changes greater than a second threshold. For example, where the sensor is a capacitive humidity sensor, it is sensitive to the amount of water in a given volume (between the capacitor's plates). Recognizing this relationship and the effect of pressure, a correlation can be expected between total pressure and the capacitance (lower pressure causes a lower capacitance to be measured if specific humidity remains unchanged). As such, by monitoring whether capacitance changes less than a threshold in the presence of pressure changes at the sensor, it is possible to identify whether the sensor is functioning, including whether the gain between a change in pressure and a change in humidity/capacitance is as expected, and from that identify sensor degradation.
In another example, some of the above issues may be addressed by a method comprising generating an indication of humidity from the humidity sensor while varying pressure over a range, the humidity sensor positioned upstream of a compressor in an engine intake system; and indicating degradation of the humidity sensor when the indication of humidity changes by less than a threshold amount with the variation in pressure.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.